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Thread: BUG #13775: CREATE RULE documentation seems to be wrong under ON SELECT TO DO INSTEAD SELECT

BUG #13775: CREATE RULE documentation seems to be wrong under ON SELECT TO

DO INSTEAD SELECT
From
needthistool@gmail.com
Date:
The following bug has been logged on the website:

Bug reference:      13775
Logged by:          Seldom
Email address:      needthistool@gmail.com
PostgreSQL version: 9.2.14
Operating system:   Linux 4.1.10-17.31.amzn1.x86_64 #1 (~RHEL 4.8.3-9)
Description:

>From the docs:

"Thus, an ON SELECT rule **effectively** turns the table into a view, whose
visible contents are the rows returned by the rule's SELECT command **rather
than whatever had been stored in the table** (if anything). It is
**considered better style** to write a CREATE VIEW command than to create a
real table and define an ON SELECT rule for it." {'**' emphases added}

As far as I can tell, the above is no longer correct whatsoever; at least on
my own server, the CREATE RULE "_RETURN" syntax **actually** turns the table
into a view, dropping the table in question and creating a new view which
has the same name and columns as the table (undocumented-ly possible only if
there are no indexes, triggers, or foreign-key constraints on the current
table, in addition to no rows, which makes the "if anything" remark pretty
misleading).

The documentation /suggests/ that CREATE VIEW ... be used in place of ON
SELECT TO <table> DO INSTEAD SELECT ... however unless I'm mistaken, this is
actually not a **choice**, merely obfuscated sugar for CREATE VIEW. It does
not appear possible to shadow a table with an alternate selection-clause
which makes the real contents of the table appear different in some way (for
instance, adding a default sorting or if one wasn't specified or such).

I don't know what the consensus is with regard to what this syntax may do in
future, but right now, it appears the docs simply do not relate to the
actual behavior of the codebase.

Re: BUG #13775: CREATE RULE documentation seems to be wrong under ON SELECT TO

DO INSTEAD SELECT
From
Tom Lane
Date:
needthistool@gmail.com writes:
>> From the docs:

> "Thus, an ON SELECT rule **effectively** turns the table into a view, whose
> visible contents are the rows returned by the rule's SELECT command **rather
> than whatever had been stored in the table** (if anything). It is
> **considered better style** to write a CREATE VIEW command than to create a
> real table and define an ON SELECT rule for it." {'**' emphases added}

I don't see anything particularly wrong with those statements...

> It does
> not appear possible to shadow a table with an alternate selection-clause
> which makes the real contents of the table appear different in some way (for
> instance, adding a default sorting or if one wasn't specified or such).

I don't see anything in the docs that suggests that that's supported,
either.

The only reason creating an ON SELECT rule is still supported at all is
that (1) that used to be how pg_dump dumped views, and (2) that's still
how pg_dump dumps views in some corner cases involving circular
dependencies.  We could get rid of (2); but in view of the need for
backwards compatibility with old dump files, it's hard to see when we
could drop the syntax entirely, so nobody's bothered.  In the meantime,
though, there is no expectation that we'd ever support ON SELECT rules
that weren't a weird spelling of CREATE VIEW, and I don't see anything
in the documentation that suggests differently.

            regards, tom lane